1. Industrial Field of the Invention
The present invention relates to a flexible plane heater and, more particularly, to a self-temperature control flexible plane heater.
2. Description of the Prior Art
A compound in a system of conductive particles and polyethylene glycol exhibits a certain switching characteristic in a relation between temperature and electric resistance (i.e., when the temperature increases, a value of the resistance abruptly increases at a threshold temperature). A self-temperature control heater making use of this characteristic has been suggested by the inventors of the present application, and already known, such as disclosed in EP-A1-0219678, U.S. Pat. No. 4,629,584, and U.S. Pat. No. 4,780,247. In addition, it has been reported from a study that this performance of self-temperature control is attributed not to thermal expansion of volume of the compound in such a system but to electron displacement through layers of polyethylene glycol which are interposed between the conductive particles ("Polymer", vol. 29; p. 526, 1988). According to this report, the formation of crystalline phase in polyethylene glycol is requisite in order to enable the performance of self-temperature control. In effect, it has been also concluded from the investigation up to the present by the inventors of the present application that crystalline phase of the compound is essential for performing the self-temperature control.
U.S. Pat. No. 4,780,247 mentioned above has also suggested that, when an amount of polyethylene glycol whose molecular weight is about 100 to 50,000 is controlled for mixing, switching temperature can be desirably varied and set within a range of about 5.degree. to 70.degree. C. In this manner, it has been progressively proved that the compound includes an excellent characteristic to serve as a heater, e.g., a heater panel for heating at 50.degree. C. or more and is of great value in practical use.
However, high polymers which contain a large number of crystalline phases (whose degree of crystallinity is high) ordinarily exhibit high brittleness and lack flexibility. For the reason, the conventional self-temperature control heater of the compound in the conductive-particles/polyethylene-glycol system has usually included polyethylene glycol whose molecular weight is about 600 to 6,000, and consequently, not only shape recoverability but also flexibility has been still unfavorable.
Polyethylene glycol is in a liquid state at the normal temperature when the molecular weight is small (M&lt;600), and as the molecular weight increases, polyethylene glycol is changed into a wax state and further proceeds into a solid state. When polyethylene glycol in the solid state is shaped into a film, the film is relatively brittle in case of the low molecular weight. But if the molecular weight is over 100,000, such a film becomes flexible. Polyethylene glycol having a molecular weight of 600 to 6,000 which has been used for melting snow or heating takes the most remarkable switching effect, but on the other hand, there has been a problem that this kind of polyethylene glycol has high crystallinity, resulting in that only brittle films will be produced.
In the present invention, the inventors have succeeded in developing a plane heater whose flexibility is realized by using super high polymeric polyethylene glycol so as to change crystalline phase of polyethylene glycol, and which plane heater also performs desirable self-temperature control In this specification, any chemical substance containing a chain of --(CH.sub.2 --CH.sub.2 --O).sub.n -- as a unit structure is referred to as polyethylene glycol.